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North America Dynamics and Western U.S. Tectonics Eugene D

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North America Dynamics and Western U.S. Tectonics Eugene D North America Dynamics and Western U.S. Tectonics Eugene D. Humphreys and David D. Coblentz1 Department of Geological Sciences, University of Oregon, Eugene, Oregon, USA 1Now at Las Alamos National Lab, New Mexico, USA Interest and controversy exists on the origin of forces that move and tectonically deform plates, especially regarding the relative importance of loads applied to the plate margins, base and those created internally, such as by high elevations. To quantify these loads, we predict the observed stress field by applying loads to a 2D North America plate model, finding that boundary loads are most important, followed by internal and basal loads. Craton-root basal drag of ~4MPa opposes absolute plate motion, compared to basal tractions elsewhere that average ~0.4MPa, suggesting that North America is separated from a relatively static deep Earth mantle by a weak asthenosphere. San Andreas shear (~1.5TN/m), gravitational collapse and southern Cascadia pull all contribute importantly to western U.S. deformation; the region also is relatively weak. Important future work includes incorporating 3D plate structure onto global flow calculations and including the global set of plates. 1. Introduction The tectonic motion and deformation of a plate are responses to the forces acting on it. The classical view of the forces responsible for plate motion has edge forces driving rigid plates over a weak and relatively static asthenosphere that acts to resist plate motion. Recently, this view has been challenged by some global geodynamicists, who advocate a model with mantle flow (driven primarily by ocean lithosphere sinking in the lower mantle) driving plates from below. The forces driving plate motion also generate tectonic stresses responsible for earthquakes and lithospheric deformation. A point of particular significance and continued contention is the relative and absolute importance of loads applied to the plate margin, base, and interior. On a smaller scale, knowledge of stress magnitude on plate boundaries bears directly on fault stress levels, faulting mechanics and the distribution of stress through the crust and mantle lithosphere. The North American plate offers good opportunity for making progress on understanding the origin and magnitude of plate forces, given the relatively simple and well-understood nature of the boundary loads acting on the plate and the abundance and large spatial distribution of observed intraplate stress indicator data. In this paper, North America stress indicators are used to evaluate the magnitudes of the various tectonic loads acting on this plate. Our modeling of plate stress is presented in an appendix. This modeling incorporates approximates for all the important tectonic loads acting on the North American plate, and the goal is to estimate the relative and absolute importance of these loads to the horizontal stresses within North America. Fig. 1 illustrates the origin of the various loads (i.e., forces) acting on a plate, and by force balance these loads sum to zero (see also Lithgow-Bertelloni and Guynn [2004] for a good discussion on the basic physics of plate stresses). These loads can be grouped into categories: edge forces, basal tractions, and internal loads arising from crustal and uppermost mantle density structure. Edge forces are created directly by plate-to- plate interaction across shared plate boundaries. Stress continuity requires that two plates in contact apply equal but opposite forces on each other across the shared boundary. Stress continuity holds across any chosen surface, so that isolating the loads acting on plate boundaries can be seen as a convenient way of isolating a plate from neighboring plates. Plate density structure not only creates surface elevation through isostasy, the resulting gravitational potential energy (GPE) plays a role in creating horizontal stresses within the plate. These stresses result from the depth-integrated vertical stress created by the weight of the overburden rock [e.g., Fleitout, and Froidevaux, 1982]; the horizontal gradient of this integral through the lithosphere creates horizontal deviatoric stresses available for tectonic processes. When these stresses drive extension the processes is commonly referred to as gravitational collapse. The stress contribution created by GPE is both one of the more difficult quantities to estimate, and the one plate load that is estimated with an actual magnitude (i.e., not simply relative information). Because GPE yields absolute units, it provides the reference needed to scale the applied loads to absolute units of stress. Tractions acting on the base of a plate result from viscous flow in the Earth’s interior, either that created by sub-lithospheric density structure (e.g., subducted slabs) or by plate motion with respect to the deeper Earth. Basal tractions on a plate are a result of viscous flow in the Earth’s interior, either that created by sub-lithospheric density structure (e.g., subducted slabs) or by relative plate motion with respect to the deeper Earth. Basal tractions can be either vertical, giving rise to dynamic topography, or horizontal. Each contributes importantly to horizontal stresses within a plate. The importance of vertical tractions can be understood by recognizing that the resulting dynamic topography contributes to the GPE. If the distinction between isostatic and dynamic topography is made by distinguishing between the effects of lithospheric and sub-lithospheric density structure [e.g., Panasyuk and Hager, 2000], then “ridge push” gravity sliding is a result of dynamic topography. 1.1 The Forces That Move Plates The intact motion of plates away from spreading centers and toward subduction zones led early workers to conclude that plates are bounded by relatively weak boundaries and basal conditions and that gravitational forces both in the form of plates sliding away from ridges (“ridge push”) and the pull of subducted slabs (“slab pull”) are most important plate-driving mechanisms [e.g., Elsasser, 1969; McKenzie, 1969; Morgan, 1972; Minster et al., 1974]. The plate velocity models that quickly followed [Forsyth and Uyeda 1975; Chapple and Tullis, 1977] found that slab pull was the most important force applied to plates, and that tractions on the base of continents slightly resist motion. More recent and physically complete Earth models support the conclusions that a weak asthenosphere results in plates that are weakly coupled to the underlying mantle [Morgan et al., 1995] and in addition that weak faults bound plates [Zhong and Gurnis, 1996; Zhong et al., 1998]. Models that include mantle flow driven by the sinking slabs (as inferred from geoid data, tomography or subduction history) conclude that subduction-related mantle flow provides the dominant driving force [Zhong et al., 2000; Lithgow-Bertelloni and Richards, 1998; Becker and O'Connell, 2001; Forte and Mitrovica, 2001]. Within the framework of rigid plates, “slab pull” and slab-driven flow produce similar effects and are difficult to distinguish. In the former, upper mantle slab applies load near the subduction margin and lower mantle slab has little effect, whereas in the latter, flow driven by lower mantle slab can dominate plate loading [Lithgow-Bertelloni and Richards, 1998; Becker and O'Connell, 2001; Forte and Mitrovica, 2001] and basal tractions are distributed more widely over the base of a plate. Plate stress models have a potential to resolve the distribution of the loads applied to plates. However, earlier models [e.g., Sykes and Sbar, 1973; Solomon et al., 1975; Richardson et al., 1976] did not incorporate space-varying basal tractions that would distinguish between basal and plate margin loading. The more recent modeling of Steinberger et al. [2001], which incorporates global flow explicitly into the consideration of plate stress, indicates that global stress orientation data can be explained well by either model, but that stress magnitude and the details of stress orientation differ between these two models, permitting observationally based discrimination. The location of the imaged subducted Farallon slab beneath eastern U.S. offers a clue on the nature of lower mantle flow beneath North America. Grand et al. [1997] and Bunge and Grand [2000] find that the location of subducted Farallon slab in the lower mantle beneath eastern U.S. is predicted well by models that have the Farallon slab dropping passively from the western North America paleo-subduction zone, which then is overridden by the moving North America plate. The portion of the Farallon slab thought to have been involved in the Laramide orogeny is found displaced ~1500 km east Bunge and Grand [2000], as predicted by flat-slab subduction during the Laramide [e.g., Coney and Reynolds, 1977; Spencer, 1996]. The ability of these simple models to account for slab location suggests that beneath North America no major drift of the lower mantle has occurred relative to a global reference. To date, 3D modeling of the effects of global flow on plate stress has not included lateral variations in viscosity, such as that exhibited by deep craton roots. If coupling at the root is great compared to the rest of the plate, then stresses around the craton should indicate whether coupling is drive or drag in nature. Wesnousky and Scholz [1980] used this reasoning to argue for cratonic drag as North America moves over a relatively static mantle. Fouch and Fisher [2000] made a similar suggestion based on mantle anisotropy, which they interpreted to indicate asthenosphere flow deflecting around the North American craton. In contrast, Bolkelmann [2002a, b] used inferred anisotropic fast-axis dip derived from P-wave travel-time delays to argue that mantle flow has driven North America. 1.2 Origin Of North America Intraplate Stress Field The causes of plate stress and resulting tectonic activity in various portions of North America is the subject of many papers. This is especially true in the western U.S., where the causes of deformation occurring broadly over much of the western U.S. have been of particular interest.
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